This invention generally relates to quadrifilar antennas used for radiating or receiving circularly polarized waves. More particularly, this invention relates to an improved quadrifilar antenna and its feed system for coupling signals of equal magnitude and 90 degrees out of phase to one end of the antenna, and to a method of manufacturing such an antenna.
It is well known that helical antennas comprising a plurality of resonant elements arranged around a common axis are particularly useful in ground links with orbiting satellites or in mobile/relay ground links with geosynchronous satellites. Due to the arrangement of the helical elements, the antenna exhibits a dome-shaped spatial response pattern and polarization for receiving signals from satellites. This type of antenna is disclosed in xe2x80x9cMultielement, Fractional Turn Helicesxe2x80x9d by C. C. Kilgus in IEEE Transactions on Antennas Propagation, July 1968, pages 499 and 500. This paper teaches, in particular, that a quadrifilar helix antenna can exhibit a cardioid characteristic in an axial plane and be sensitive to circularly polarized emissions.
One type of prior art helical antenna comprises two bifilar helices arranged in phase quadrature and coupled to an axially located coaxial feeder via a split tube balun for impedance matching. While antennas based on this prior design are widely used because of the particular response pattern, they have the disadvantage that they are extremely difficult to adjust in order to achieve phase quadrature and impedance matching, due to their sensitivity to small variations in element length and other variables, and that the split tube balun is difficult to construct. As a result, their manufacture is a very skilled and expensive process.
Therefore, there is a need for a quadrifilar antenna having a predetermined input impedance which could be manufactured on a production basis without the need for adjustment and costly individual tuning. Further, there is a need to provide a quadrifilar antenna having a simplified feed arrangement that avoids the complexities of conventional folded, stepped or split shield baluns.
The subject invention herein solves all of these problems in a new and unique manner which has not been part of the art previously. Some related patents are described below:
U.S. Pat. No. 5,635,945 issued to McConnell et al on Jun. 2, 1993
This patent is directed to a quadrifilar helix antenna comprising four conductive elements arranged to define two separate helically twisted loops, one slightly differing in electrical length than the other, to define a cylinder of constant radius supported by itself or by a cylindrical nonconductive substrate. The two separate helically twisted loops are connected to each other in such a way as to constitute the impedance matching, electrical phasing, coupling and power distribution for the antenna.
U.S. Pat. No. 5,191,352 issued to S. Branson on Mar. 2, 1993
This patent is directed to a quadrifilar antenna comprising four helical wire elements shaped and arranged so as to define a cylindrical envelope. The helical wires are mounted at their opposite ends by first and second printed circuit boards having coupling elements in the form of plated conductors which connect the helical wires to a feeder or semi-rigid coaxial cable on the first board, and with each other on the second board. The conductor tracks are such that the effective length of one pair of helical wires and associated impedance elements is greater than that of the other pair of helical wires, so that phase quadrature is obtained between the two pairs.
U.S. Pat. No. 4,008,479 issued to V. C. Smith on Feb. 15, 1977
This patent is directed to a dual-frequency circularly polarized antenna. The antenna comprises a longitudinal cylindrical non-conductive member supported at its top by four conductors each extending transversely from a center coaxial line. Two sets of the antenna conductors are attached to the non-conducting cylinder in a configuration of equally longitudinally spaced spirals. The two sets of conductors are conductively connected by pins such that one set corresponds to a half wavelength at one frequency and the other set corresponds to a half wavelength at another frequency.
U.S. Pat. No. 3,623,113 issued to I. M. Falgen on Nov. 23, 1971
This patent is directed to a tunable helical monopole antenna. The tunable helical monopole antenna comprises a winding having both an upper portion and a lower portion which are symmetrically substantially identical to each other. Connected to each end of the winding halves are cylindrical terminal dipole elements and connected to these terminal elements are shorting fingers. By synchronously moving the shorting fingers, the respective helical windings are effectively shorten or lengthen for tuning purposes.
U.S. Pat. No. 5,255,005 issued to Terret et al. on Oct. 19, 1993
This patent is directed to a dual layer resonant quadrifilar helix antenna. The antenna comprises a quadrifilar helix formed by first and second bifilar helices positioned orthogonally and excited in phase quadrature. Additionally, a second quadrifilar helix is coaxially and electromagnetically coupled to a first quadrifilar helix.
U.S. Pat. No. 4,148,030 issued to P. Foldes on Apr. 3, 1979
This patent is directed to a combination helical antenna comprising a plurality of tuned helical antennas which are coaxially wound upon a hollow cylinder, whereby the antennas are collocated. The antenna further comprises a printed circuit assembly having thin metal dipoles of the type used in a microwave strip line. The thin metal dipoles are resonating elements that are coupled to each other in a manner similar to end-fire elements of a microstrip filter.
While the basic concepts presented in the aforementioned patents are desirable, the apparatus employed by each to produce a quadrifilar antenna are mechanically far too complicated to render them as an inexpensive means of achieving an antenna having a predetermined input impedance which could be manufactured on a production basis without the need for adjustment and costly individual tuning and still present desired radiation characteristics during operation.
A quadrifilar antenna for use in satellite communications comprises four conductive elements arranged to define two separate helical pairs with both pairs being open circuited at one end, one pair slightly differing in electrical length than the other, to define a cylinder of constant radius supported by itself or by a cylindrical non-conductive substrate. The two separate helical pairs are connected to each other in such a way as to constitute the impedance matching, electrical phasing, coupling and power distribution for the antenna. In place of a conventional balun, the antenna is fed at a tap point on one of the conductive elements determined by an impedance matching network which connects the antenna to a transmission line. The matching network can be built with distributed or lumped electrical elements and can be incorporated into the design of the antenna.
Therefore, it is an object of the present invention to provide a simple matching network where the inductance of the conductor leading to the tap point is tuned out by a capacitor connected to the transmission line used to transfer radio frequency signals to and from the antenna.
An object of the present invention is to provide a quadrifilar antenna formed by a pair of helical elements where the coupling between the pair of helical elements is provided by a shared common current path.
A further object of the present invention is to have a quadrifilar antenna which has a simple feed method that does not require the use of conventional folded, stepped or split shield baluns.
Another object of the present invention is to provide a quadrifilar antenna formed by printed circuit boards which can be relatively accurately formed with predetermined shapes and dimensions, such that relatively little, if any, adjustment is required to obtain an antenna having the required electrical characteristics.
Yet, still another object of the present invention is to have a quadrifilar antenna which can be mass-produced to precise dimensions with high reproducibility of electromagnetic characteristics.
Still, yet another object of the present invention is to provide a quadrifilar antenna which is especially simple in construction, particularly light weight and compact in design.
A further object of the present invention is to provide a low cost antenna having a quasi-hemispherical radiation pattern of the type formed by two bifilar helices used in ground and orbital satellite telecommunication links or in mobile relay telecommunication links with geosynchronous satellites.
Another object of the present invention is to provide a method of making a radio frequency antenna having a plurality of helical elements formed through the use of alignment tabs for ease and accuracy in manufacturing.
Accordingly, it is an object of the present invention to provide an effective, yet inexpensive and relatively mechanically unsophisticated quadrifilar antenna, which is rugged yet lightweight, easily carried and used.